1. Field of the Invention
The present invention relates to nonwoven fabric.
2. Related Art
Conventionally, nonwoven fabrics are used in a variety of fields, for example, sanitary products such as disposable diaper and sanitary napkin, cleaning products such as wipers and medical products such as masks. Such nonwoven fabrics are used in many different fields, but when actually used in products of each of those fields, it is necessary that the nonwoven fabric are manufactured with the properties and structures appropriate for their intended use.
For example, nonwoven fabric can be manufactured by forming a fiber layer (fiber web) either by a dry method or a wet method, and then bonding the fibers of the fiber layer together by a chemical bond method or a thermal bond method. Methods for bonding the fibers that form the fiber layer includes a method of externally applying a physical force to the fiber layer where a plurality of needles are repeatedly inserted into the fiber layer or a method of applying a physical force where aqueous steam is injected into the fiber layer.
However, according to these methods the fibers are only entangled with each other and the orientation or arrangement of the fibers of the fiber layer or the shape of the fiber layer is not adjusted. In other words, these methods only manufacture a simple sheet-like nonwoven fabric.
For example, in the case of nonwoven fabric used as a top sheet of an absorbent article, when the liquid of an excrement is absorbed during use of the absorbent article, a nonwoven fabric having irregular portions is preferably used in order to maintain or improve the positive feeling of the article when it comes into contact with the skin of the user. Japanese Patent Publication No. 3587831 (hereinafter referred to as Patent Document 1) discloses a nonwoven fabric in which a plurality of finer layers, including fibers of different heat shrinkability, are stacked together to be thermally bonded, and irregular portions are formed on a surface by heat contraction of at least one of the fiber layers, and its manufacturing method.
In the case of the nonwoven fabric disclosed in Patent Document 1, when the irregular portions are formed in the nonwoven fabric, the plurality of fiber layers are stacked together to integrate the fibers of the fibers layers by thermal bonding, and thus the fiber density increases in areas where the fibers have been thermally bonded. As a result, permeation of a predetermined liquid of an excrement through an absorbent layer becomes difficult. In addition, when the thermally bonded area is formed into a film, rapid downward permeation of the predetermined liquid excrement also becomes more difficult.
The nonwoven fabric disclosed in the Patent Document 1 is manufactured by stacking a second fiber layer made of a thermal non-contractive fiber on one or both surfaces of a first fiber layer consisting of a fiber including a thermally contracted thermal contractive fiber. The fibers are integrated by a large number of thermally bound portions. In the thermally bound portions, the second fiber layer projects by thermal contraction of the first fiber layer to form portions of varying height.
In other words, as a plurality of fiber layers that differ from one another in their nature are necessary to form irregular portions on the surface of the nonwoven fabric, the manufacturing process is complex. When the first and second fiber layers are peeled off during thermal contraction of the thermal contractive fiber, the second fiber layer cannot form a convex portion. Thus, to prevent the first and second fiber layers from peeling, the thermally bonded portions must be securely bound. However in this situation the density of the thermally bonded portions increases, a film is formed, and rapid permeation of the liquid of the excrement becomes difficult. These are the problems to be solved by the present invention.